One of the methods of producing a metal powder known in the art is an atomization process. There are two types of atomization processes: a water atomization process in which a high-pressure water jet is made to impinge on a molten metal stream to produce a metal powder; and a gas atomization process in which, instead of a water jet, an inert gas is made to impinge on a molten metal stream.
In a water atomization process, a water-atomized metal powder is produced by dividing a molten metal stream into a powdered metal (metal powder) with a water jet ejected through nozzles and cooling the powdered metal (metal powder) with the water jet. On the other hand, in a gas atomization process, an atomized metal powder is produced by dividing a molten metal stream into a powdered metal (metal powder) with an inert gas ejected through nozzles and, generally, cooling the powdered metal (metal powder) by dropping the powdered metal into a tank containing water or a drum containing swirling water disposed below the atomizing device.
Recently, a reduction in the iron losses of motor cores for electric vehicles, hybrid vehicles and the like has been anticipated from the viewpoint of energy conservation. While motor cores are produced using multilayers of electromagnetic steel sheets, attention is being focused on motor cores formed of a metal powder (electromagnetic iron powder), which allows a high degree of flexibility in designing the shapes of the motor cores. To reduce the iron losses of such motor cores, it is necessary to reduce the iron loss of a metal powder constituting the motor cores. To reduce the iron loss of the metal powder, it is considered to be effective to change the metal powder into an amorphous state. To produce an amorphous metal powder by an atomization process, however, it is necessary to cool the metal powder that is in a high-temperature condition including a molten state at a considerably high cooling rate to prevent crystallization of the metal powder.
Accordingly, there have been proposed several methods of rapidly cooling a metal powder.
For example, Japanese Unexamined Patent Application Publication No. 2010-150587 describes a method of producing a metal powder in which scattered molten metal particles are cooled and solidified to form a metal powder. The rate at which the molten metal particles are cooled until they solidify is 105 K/s or more. In the technique described in JP '587, the above cooling rate is achieved by bringing the scattered molten metal particles into contact with a stream of a cooling liquid generated by passing the cooling liquid along the inner wall of a cylindrical body in a spiral. It is described that the flow rate of the stream of the cooling liquid, which is generated by passing the cooling liquid in a spiral, is preferably 5 to 100 m/s.
Japanese Examined Patent Application Publication No. 7-107167 describes a method of producing a rapidly solidified metal powder. In the technique described in JP '167, a cooling liquid is fed from the outer periphery of the top end of a cylindrical portion of a cooling container having a cylindrical inner periphery in the circumferential direction to flow downward along the inner periphery of the cylindrical portion in a spiral. The cooling liquid forms a laminar, spiral cooling-liquid layer having a cavity at the center due to the centrifugal force generated by the spiral stream of the cooling liquid. A molten metal is fed to the inner periphery of the spiral cooling-liquid layer and rapidly solidified. This enables a high-quality, rapidly solidified powder to be produced with a high cooling efficiency.
Japanese Patent No. 3932573 describes an apparatus that produces a metal powder by a gas atomization process and includes a gas jet nozzle through which a gas jet is made to impinge on a molten metal stream to divide the molten metal stream into molten metal droplets and a cooling cylinder including a layer of a cooling liquid that flows downward along the inner periphery of the cylinder in a spiral. In the technique described in JP '573, the molten metal is divided in two stages by using the gas jet nozzle and the spiral cooling-liquid layer. This enables a fine, rapidly solidified metal powder to be produced.
Japanese Patent No. 3461344 describes a method of producing amorphous metal fine particles. In that method, a molten metal is fed into a liquid coolant such that a steam film that covers the molten metal is formed in the coolant, and the steam film is broken to bring the molten metal into direct contact with the coolant. This induces boiling caused due to natural nucleation. While the molten metal is divided into particles with the power of the pressure wave resulting from the boiling, the molten metal particles are rapidly cooled and changed into an amorphous state. Thus, amorphous metal fine particles are produced. It is described that the steam film that covers the molten metal can be broken by controlling the temperature of the molten metal fed into the coolant such that, when the molten metal is brought into direct contact with the coolant, the temperature of the molten metal at the interface between the molten metal and the coolant is equal to or lower than the minimum film boiling temperature and equal to or higher than the spontaneous nucleation temperature or by using ultrasound.
Japanese Patent No. 4793872 describes a method of producing fine particles. In that method, a molten material is fed into a liquid coolant in the form of droplets or a jet stream while the temperature of the molten material is such that the molten material has a temperature equal to or more than the spontaneous nucleation temperature of the liquid coolant and is in a molten state when contacting the liquid coolant. Furthermore, the difference in relative velocity between the molten material and a stream of the liquid coolant at the time the molten material is fed into the stream of the liquid coolant is controlled to 10 m/s or more. This causes the steam film formed around the molten material to be forcibly broken and boiling to occur due to spontaneous nucleation. Thus, the molten material is formed into fine particles, and the fine particles are cooled and solidified. That method is said to enable materials that have been considered to be difficult to be formed into fine particles and changed into an amorphous state to be formed into fine particles and changed into an amorphous state.
Japanese Patent No. 4784990 describes a method of producing a functional member, the method including a step in which a raw material prepared by adding a functional additive to a base material is molten and fed into a liquid coolant to cause steam explosion, which enables the molten raw material to be formed into fine particles, and the fine particles are cooled and solidified at a controlled cooling rate to form homogeneous functional fine polycrystalline or amorphous particles free from segregation and a step in which the functional fine particles and fine particles of the base material used as raw materials, are solidified to form a functional member.
In general, it is difficult to bring the surface of a molten metal into perfect contact with cooling water when the hot molten metal is brought into contact with the cooling water to rapidly cool the molten metal. This is because the cooling water vaporizes upon coming into contact with the surface (surface to be cooled) of the hot molten metal and forms a steam film between the surface to be cooled of the molten metal and the cooling water, that is, the cooling water is brought into the film boiling state. The presence of the steam film inhibits facilitation of cooling of the molten metal.
In the techniques described in JP '587, JP '167 and JP '573, attempts have been made to remove a steam film formed around metal particles by feeding a divided molten metal into a layer of a cooling liquid formed of a spiral stream of a cooling liquid. However, if the temperature of the metal particles is high, film boiling is likely to occur in the cooling-liquid layer. In addition, since the metal particles fed into the cooling-liquid layer move together with the cooling-liquid layer, the difference in relative velocity between the metal particles and the cooling-liquid layer is small. This makes it difficult to prevent a film boiling state.
In the techniques described in JP '344, JP '872 and JP '990, a steam film covering a molten metal is broken with the power of steam explosion by which the film boiling state is serially into the nucleate boiling state to produce amorphous metal fine particles. Removing a steam film formed during film boiling with the power of steam explosion is an effective approach. However, to cause steam explosion by making the film boiling state into the nucleate boiling state, as is clear from the boiling curve illustrated in FIG. 4, it is necessary to at first at least reduce the surface temperature of the metal particles to the MHF (minimum heat flux) point or less. The graph shown in FIG. 4 is referred to as “boiling curve” that schematically illustrates the relationship between the cooling capacity of a water coolant (cooling water) and the surface temperature of a material to be cooled. As illustrated in FIG. 4, if the surface temperature of metal particles is high, cooling of the metal particles to the MHF temperature is performed in the film-boiling region. The intensity of cooling of the metal particles performed in the film-boiling region is low because of the presence of steam films interposed between the surfaces to be cooled of the metal particles and the cooling water. Accordingly, if the metal particles are cooled from a temperature equal to or more than the MHF temperature to produce an amorphous metal powder, there is a problem that the cooling rate of producing amorphous is insufficient.
It could therefore be helpful to provide a method of producing an atomized metal powder that enables rapid cooling of the metal powder to be achieved and an amorphous metal powder to be produced.